Vulnerability analysis and new tools for climate change adaptation of marine and lagoon systems

Climate change (CC), particularly sea warming, and marine heat waves (MHWs), are affecting coastal marine ecosystems around the world on different hierarchical levels of ecosystem organization. In this context, the Mediterranean Sea can be considered an "open-air laboratory", as it is universally recognized as a "miniature ocean" which. Due to its geographical and geomorphological characteristics, it already exhibits the changes expected in the global ocean by 2050 and beyond. Among its coastal marine environments, coastal lagoons play a crucial ecological role due to their strategical position between land and sea. However, due to their characteristics, they are particularly vulnerable to climate change, considering them as "sentinel systems" (Eisenreich, 2005; Brito et al., 2012). In this context, the aim of my thesis was to assess the impact of sea warming and marine heatwaves on species, communities, and functions of Mediterranean marine-coastal ecosystems, with a particular focus on coastal lagoon environments. I first focused on the impact of sea warming on species of ecological and economic interest along the coasts of Sardinia. The construction of monthly thermal habitat suitability maps for a reference period (1987-2019) and future sea warming scenarios up to 2050 (SSP2-4.5 and SSP5-8.5) for each species, allowed me to identify how the potential thermal habitat suitability could change for species identifying which ones may be most impacted and when. I then investigated how marine heat waves can affect the quantity, composition and degradation rates of sedimentary organic matter in a Mediterranean coastal lagoon (Santa Gilla). Through simulation in benthocosms of marine heat waves of different intensities, I was able to highlight their impact on sedimentary organic matter of coastal lagoons, showing a possible alteration of resources for benthic consumers and the C storage capacities of lagoons. I then deepened this knowledge by evaluating how sedimentary type, and consequently organic loads, can influence the impact of marine heat waves on benthic metabolism comparing two coastal lagoons located in different semi-enclosed seas (the Mediterranean and the Baltic) and at different latitudes (low and high). Through integration between mesocosm experiments and incubation techniques for the measurement of gas and nutrient fluxes, I highlighted how the intensity of heat waves and the type of sediment, as well as the difference in organic loads, can influence the intensity and type of alteration of benthic metabolism ("oligotrophy" vs "eutrophy"). I then observed in the field the seasonal and spatial variability and the effects of a dystrophic crisis on a lagoon in Sardinia, events caused by nutrient enrichment but exacerbated by temperature increases, on the quantity, quality and composition of sedimentary organic matter as well as on the benthic communities of meiofauna. The field assessment helped me to understand the most vulnerable areas to the impact and cascading responses of the change in trophic status on environmental quality. Finally, I integrated all the knowledge acquired in all the research activities within impact chains, logical-formal tools useful for climate change risk assessment. This allowed me to schematize the logical flow of cause and effect between hazards (surface water warming and heat waves) and vulnerability (elements of sensitivity and/or capacity of species and ecosystems) and possible impacts on species and ecosystems. Although many aspects still need to be explored, my PhD thesis (through the use of various methodologies, including literature review, modeling approaches, experiments on the mesocosm and field investigations) has contributed to expanding the current knowledge on the assessment of the vulnerability and impacts of climate change.